Don't wish to be rude but if you have to ask a question like that then maybe your not up to the task.
The two I've done both use the same size bottle but have got a different offset as the first (shown in my album) was a standard full length rifle and the second was fitted with the barrel from the FA version which was 4" longer and came with the barrel shroud which is 3mm bigger in diameter so the bottle had to be mounted 1.5 mm lower.
The offset required would have to take into account the barrel and bottle diameter and the amount of cap you wish between the two, I went for 2mm.
The stock was a Paul Wilson with a little " tweaking " by myself, the rest was all my own work.
I'll have to get round to adding a picture of my second conversion to my album which I fitted into an EV stock
I'm shooting at Emley on Saturday and Redfearns on Sunday but fell free to look me up on the 31st at to club